1. Technical Field
This invention relates generally to a variable stiffness structural member for use in the wing of an aircraft, and more particularly to a rotatable structural member that provides selectivity in the amount of strength and stiffness that the structural member contributes to the wing structure.
2. History of Related Art
An arrangement of spars, ribs, and wing skin typically provide the primary load-carrying structure of an aircraft wing. This assembled structure, called the wing structural box, provides the primary source of structural stiffness for modern, high-performance, monocoque wings. The wing structural box determines the load-carrying capability of the wing and the structural stiffness of the wing's aeroelasticity.
The aeroelasticity of a conventional wing with fixed stiffness dictates that as the wing speed increases, the aerodynamic moments of the wing air loads progressively become stronger and may reach a value that exceeds the torsional stiffness of the wing. Large aerodynamic moments become especially troublesome for wings that provide aircraft control functions by means of leading and/or trailing edge control surfaces, or flaps. High aerodynamic moments may cause a loss in the control forces that the flaps are able to provide to the aircraft if the aeroelastic losses become so great that control capability is compromised. The air speed at which this occurs is classically referred to as the aileron or control reversal air speed. If the air speed is increased past the reversal air speed, the force perturbations begin to return but are opposite in sign to those generated below the reversal air speed.
The aeroelastistic contributions to the loss of the control functions may be so severe that the utility of using wing-mounted ailerons is marginal for high performance transonic aircraft of any size. The control function is usually recovered by the use of a much less efficient all-movable horizontal tail surfaces. Aircraft designers would like to employ higher aspect ratio wing platforms for increased range or speed without the need for roll control from horizontal tails.
The present invention is directed to overcoming the problems described above. It is desirable to have a wing structural box in which the stiffness of the wing can be varied to turn the aeroelastistic control function loss into an aircraft control function asset. A variable stiffness wing permits the use of more efficient wing plan forms and the removal of horizontal tail control surfaces. Moreover, it is desirable to have at least one of the structural members, i.e., a spar or rib, that has selectively variable stiffness so that the amount of strength and stiffness that the spar or rib contributes to the wing structure can be modulated. It is also desirable to have a wing structural box in which the wing torsional stiffness can be varied with changes in air speed, so that in all speed ranges the wing ailerons produce the necessary control forces to properly maneuver the aircraft, with aircraft roll control being desirably provided by using only the wing-aileron surfaces.